The present invention relates to an improvement in a data compression device for a facsimile apparatus or like picture processing apparatus.
In facsimile communications, for example, so-called redundancy reduction coding is performed on picture signals with a view to shortening the transmission time. Generally, two different modes are available for redundancy reduction coding, i.e., one-dimensional mode wherein picture signals are coded by a processing which occurs with respect to the main scanning line direction only, and two-dimensional mode wherein signals are processed taking into account the direction of subscanning lines as well (inclusive of two-dimensional batch coding and two-dimensional sequential coding). It is generally accepted that the two-dimensional coding mode offers a greater compression effect than the one-dimensional coding mode.
Nevertheless, the compression efficiency attainable with one-dimensional coding is sometimes greater than that attainable with two-dimensional coding such as at the border between a character line and an interlinear space in an ordinary document. In light of this, there has been proposed a coding system which determines a correlationship between adjacent scanning lines and selectively combines the one-dimensional and two-dimensional coding modes based on the determined correlationship, as disclosed in Japanese Patent Laid-Open Publication No. 54-12515.
The above-described coding system offers a significant improvement in compression efficiency over the simple two-dimensional coding system. However, since the selective adoptation of the two different coding modes requires division, the operation is intricate and consumes a disproportionate period of time.